The invention relates to novel vinylidene compounds and ethyl compounds of the formula I 
in which
R1 and R2 are each, independently of one another, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted monosubstituted by CN or CF.3 or at least monosubstituted by halogen, where one or more CH2 groups in these radicals may also, in each case independently of one another, be replaced by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94⋄xe2x80x94 xe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94Oxe2x80x94 in such a way that O atoms are not linked directly to one another,
A1 (a) is a tarns-1,4-cyclohexyl radical, in which, in addition, one or more non-adjacent CH2 groups may be replaced by xe2x80x94Oxe2x80x94 and/or xe2x80x94Sxe2x80x94,
(b) a 1,4-phenylene radical, in which, in addition, one or two CH groups may be replaced by N,
(c) a 1,4-cyclohexenylene radical,
(d) a radical from the group consisting of 1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl, naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and 1,2,3,4-tetrahydronaphthalene-2,6-diyl,
where the radicals (a), (b) and (c) may be monosubstituted or polysubstituted by CN or fluorine,
Z1, Z2 
or Z3 is each, independently of one another, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94(CH2)4xe2x80x94 or a single bond, 
L1 to L6 are each, independently of one another, H or F, where at least one of the radicals L1 and L2 is F or L3 to L6 are F and the other radical L1 or L2 is H or F and the other radicals L3 to L6 are H or F,
m is 0 or 1, and
n is 1 or 2,
with the proviso that at least one bridge Z1, Z2 or Z3 is 
The invention furthermore relates to the use of these compounds as components of liquid-crystalline media, and to liquid-crystal and electro-optical display elements which contain the novel liquid crystalline media.
The compounds of the formula I can be used as components of liquid-crystalline media, in particular for displays based on the principle of the twisted cell, the guest-host effect, the effect of deformation of aligned phases or the effect of dynamic scattering.
The compounds of the formula I are distinguished by clearly negative anisotropy of the dielectric constant and are aligned in an electric field with their longitudinal molecular axes perpendicular to the field direction. This effect is known and is utilized to control the optical transparency in various liquid-crystal displays, for example in liquid-crystal cells of the light-scattering type (dynamic scattering), of the DAP (deformation of aligned phases) type, of the ECB (electrically controlled birefringence) type or of the guest-host interaction type.
Compounds of the formula I are furthermore suitable as components of chiral tilted smectic phases. Chiral tilted smectic liquid-crystalline phases having ferroelectric properties can be prepared by adding a suitable chiral dopant to base mixtures having one or more tilted smectic phases (L. A. Veresnev et al., Mol. Cryst. Liq. Cryst. 89, 327 (1982); H. R. Brand et al., J. Physique 44 (lett.), L-771 (1983). Such phases can be used as dielectrics for fast-switching displays based on the principle of SSFLC technology described by Clark and Lagerwall (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899 (1980); U.S. Pat. No. 4,367,924) on the basis of the ferroelectric properties of the chiral tilted phase.
A number of liquid-crystalline compounds having slightly negative dielectric anisotropy have already been synthesized. By contrast, relatively few liquid-crystal components having highly negative anisotropy of the dielectric constant are known. Moreover, the latter generally have disadvantages, such as, for example, poor solubility in mixtures, high viscosity, high melting points and chemical instability. There is thus a demand for further compounds having negative dielectric anisotropy which allow the properties of mixtures to be further improved for a wide variety of electro-optical applications.
Liquid-crystal compounds having negative dielectric anisotropy which contain two or three rings linked via carboxyl groups or covalent bonds and one or more side groups, such as halogen, cyano or nitro groups, are disclosed in DE 22 40 864, DE 26 13 293, DE 28 35 662, DE 28 36 086, EP 023 728, EP 0 084 194 and EP 0 364 538.
JP 05 070 382 covers, in a broad generic formula, the transmonofluoroethylene compounds claimed here. The prior art thus neither reveals to the person skilled in the art possible syntheses of the claimed compounds having negative dielectric anisotropy in a simple manner nor suggests that the compounds according to the invention have favourably located mesophase ranges and are distinguished by large negative dielectric anisotropy at the same time as low viscosity and have increased low-temperature stability.
The invention had the object of indicating stable, liquid-crystalline or mesogenic compounds having a large negative dielectric anisotropy and at the same time low viscosity and high low-temperature stability, with an unrestricted nematic phase range.
It has been found that the compounds of the formula I are eminently suitable as components of liquid-crystalline phases. In particular, they can be used to prepare stable liquid-crystalline phases having a broad mesophase range and comparatively low viscosity and high low-temperature stability.
The compounds of the formula I are furthermore suitable as components of chiral tilted smectic liquid-crystalline phases.
In addition, the provision of the compounds of the formula I generally considerably broadens the range of liquid-crystalline substances which are suitable, from various applicational points of view, for the preparation of liquid-crystalline mixtures.
The compounds of the formula I have a broad range of applications. Depending on the choice of substituents, these compounds can be used as base materials of which liquid-crystalline phases are predominantly composed; however, compounds of the formula I can also be added to liquid-crystalline base materials from other classes of compound in order, for example, to vary the dielectric and/or optical anisotropy and/or the viscosity and/or the spontaneous polarization and/or the phase ranges and/or the tilt angle and/or the pitch of a dielectric of this type.
The compounds of the formula I are furthermore suitable as intermediates in the preparation of other substances which can be used as constituents of liquid-crystalline dielectrics.
In the pure state, the compounds of the formula I are colourless and form liquid-crystalline mesophases in a temperature range which is favourably located for electro-optical use. They are very stable chemically, thermally and to light.
The invention thus relates to the compounds of the formula I. The invention furthermore relates to the use of the compounds of the formula I as components of liquid-crystalline mixtures. The invention furthermore relates to liquid-crystalline phases containing at least one compound of the formula I, and to liquid-crystal display elements containing such phases. The compounds are distinguished, in liquid-crystal mixtures, by the fact that low viscosity and an increase in the low-temperature stability are achieved, with an unrestricted nematic phase range.
In the compounds of the formulae above and below, each R1, independently of the others, is preferably straight-chain alkyl, 1E-alkenyl or 3E-alkenyl, and R2 is preferably straight-chain alkoxy having 1-5 carbon atoms, in particular OCH3 or OC2H5, furthermore alkyl having 1-5 carbon atoms, in particular CH3 or C2H5.
Preference is furthermore given to compounds of the formulae above and below in which R1 is alkoxy or oxaalkyl (for example alkoxymethyl).
Z1, Z2 and Z3 are each, independently of one another, preferably a single bond or a xe2x80x94C2H4 bridge.
The radicals R1 and R2 in the formulae above and below preferably have 2-12 carbon atoms, in particular 3-10 carbon atoms. One or two CH2 groups in R1 and R2 may also be replaced. Preferably, only one CH2 group is replaced, by xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94.
In the formulae above and below, the radicals R1 and R2 are preferably alkyl, alkoxy or another oxaalkyl group, furthermore alkyl groups in which one or two CH2 groups may be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94.
If the radicals R1 and R2 are alkyl radicals in which one (xe2x80x9calkoxyxe2x80x9d or xe2x80x9coxaalkylxe2x80x9d) or two (xe2x80x9calkoxyalkoxyxe2x80x9d or xe2x80x9cdioxaalkylxe2x80x9d) non-adjacent CH2 group(s) may be replaced by O atoms, they may be straight-chain or branched. They are preferably straight-chain, have 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly are preferably ethyl, propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy or dodecoxy.
Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl), 2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl, 1,3-dioxabutyl (=methoxymethoxy), 1,3-, 1,4- or 2,4-dioxapentyl, 1,3-, 1,4-, 1,5-, 2,4-, 2,5- or 3,5-dioxahexyl, 1,3-, 1,4-, 1,5-, 1,6-, 2,4-, 2,5-, 2,6-, 3,5-, 3,6- or 4,6-dioxaheptyl.
If the radicals R1 and R2 are an alkyl radical in which one CH2 group has been replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, the transform is preferred. This alkenyl radical may be straight-chain or branched. It is preferably straight-chain and has 2 to 10 carbon atoms. Accordingly, it is in particular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.
Compounds of the formula I containing branched radicals R1 may occasionally be of importance owing to better solubility in conventional liquid-crystalline base materials, but in particular as chiral dopants if they are optically active. Smectic compounds of this type are suitable as components of ferroelectric materials.
Branched groups of this type generally contain not more than one chain branch. Preferred branched radicals R1 are isopropyl, 2-butyl (=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl (=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methylhexoxy, 1-methylheptoxy, 2-oxa-3-methylbutyl, 3-oxa-4-methylpentyl, 4-methylhexyl, 2-nonyl, 2-decyl, 2-dodecyl or 6-methyloctoxy.
Formula I covers the racemates of these compounds and the optical antipodes, and mixtures thereof.
Of the compounds of the formula I, preference is given to those in which at least one of the radicals present therein has one of the preferred meanings given.
In the compounds of the formula I, preference is given to those stereoisomers in which the rings Cyc and piperidinyl are trans-1,4-disubstituted. Those of the abovementioned formulae which contain one or more Pyd, Pyr and/or Dio groups in each case include the two 2,5-positional isomers.
A very particularly preferred smaller group of compounds consists of those of the subformulae I1 to I5: 
Particular preference is given to compounds of the formulae I1 and I3.
In the compounds of the formula I and in the subformulae I1 and I2, R2 is preferably methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, furthermore methyl, ethyl, n-propyl, n-butyl, n-pentyl or n-hexyl.
R1 is preferably ethyl, propyl, n-butyl, n-pentyl, n-hexyl, vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 1E-octenyl, 1E-nonenyl, 1E-decenyl, allyl, 2Z-butenyl, 2Z-pentenyl, 2Z-hexenyl, 2Z-heptenyl, 2Z-octenyl, 2Z-nonenyl, 2Z-decenyl, 3E-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 3E-octenyl, 3E-nonenyl or 4E-pentenyl.
L1 and L2 are preferably fluorine. Particular preference is given to compounds of the formula I in which L1=F and L2=H or F, or L1=H or F and L2=F, and to compounds of the formula I3 in which L3=L4=H and L5=L6=F or L3-L6=F.
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in the standard works, such as Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for said reactions.
Use can also be made here of variants which are known per se, but are not mentioned here in greater detail.
Compounds according to the invention can be prepared, for example, by reacting benzene derivatives as shown in the following reaction schemes: 
Particularly preferred compounds are prepared as follows: 
The invention likewise relates to a process for the preparation of xe2x80x94C2F4-bridged alkyl compounds starting from compounds which contain a 
structural unit, conversion of the double bond into the dihydroxyl compound which contains a 
structural unit using N-methylmorpholine N-oxide and catalytic amounts of OsO4, oxidation to give the diketone 
and reaction with SF4 under reduced pressure to give the xe2x80x94C2F4xe2x80x94 structural unit 
The compounds of the formula I are prepared by methods known per se, as described in the literature (for example in standard works, such as Houben-Weyl, Methoden der Organischen Chemie [Methods of Organic Chemistry], Georg-Thieme-Verlag, Stuttgart), to be precise under reaction conditions which are known and suitable for said reactions. Use can also be made here of variants which are known per se, but are not mentioned here in greater detail.
If desired, the starting materials can also be formed in situ by not isolating them from the reaction fixture, but instead immediately converting them further into the compounds of the formula I.
Compounds of the formula I are obtainable starting from 1,2-difluorobenzene. This is metallated by known processes (for example A. M. Roe et al., J. Chem. Soc. Chem. Comm., 22, 582 (1965)) and reacted with the corresponding electrophile. This reaction sequence can be carried out a second time with the resultant 1-substituted 2,3-difluorobenzene using a suitable electrophile, giving 1,4-disubstituted 2,3-difluorobenzenes, which can be converted, if desired, into the end products in further reaction steps. 1,2-Difluorobenzene or 1-substituted 2,3-difluorobenzene is reacted with phenyllithium, lithium tetramethylpiperidine, n-, sec- or tert-butyllithium at temperatures of from xe2x88x92100xc2x0 C. to +50xc2x0 C., preferably from xe2x88x9278xc2x0 C. to 0xc2x0 C., in an inert solvent, such as diethyl ether, tetrahydrofuran, dimethoxyethane, tert-butyl methyl ether or dioxane, hydrocarbons, such as hexane, heptane, cyclohexane, benzene or toluene, or mixtures of these solvents, if desired with addition of a complexing agent, such as tetramethylethylenediamine (TMEDA) or hexamethylphosphoric triamide.
The lithium 2,3-difluorophenyl compounds are reacted with the corresponding electrophiles at from xe2x88x92100xc2x0 C. to 0xc2x0 C., preferably at below xe2x88x9250xc2x0 C. Suitable electrophiles are aldehydes, ketones, nitriles, epoxides, carboxylic acid derivatives, such as esters, anhydrides or halides, haloformic esters or carbon dioxide.
For the reaction with aliphatic or aromatic halogen compounds, the lithium 2,3-difluorophenyl compounds are transmetallated and coupled with transition-metal catalysis. The zinc (cf. DE-A 36 32 410) or titanium 2,3-difluorophenyl compounds (cf. DE-A 37 36 489) are particularly suitable for this purpose.
Suitable reducible groups are preferably carbonyl groups, in particular keto groups, furthermore, for example, free or esterified hydroxyl groups or aromatically bonded halogen atoms. Preferred starting materials for the reduction conform to the formula I, but may contain a cyclohexene ring or cyclohexanone ring in place of a cyclohexane ring and/or a xe2x80x94CHxe2x95x90CHxe2x80x94 group in place of a xe2x80x94CH2CH2xe2x80x94 group and/or a xe2x80x94COxe2x80x94 group in place of a xe2x80x94CH2xe2x80x94 group and/or a free or functionally modified (for example in the form of its p-toluenesulfonate) OH group in place of an H atom.
The reduction can be carried out, for example, by catalytic hydrogenation at temperatures between 0xc2x0 C. and about 200xc2x0 C. and at pressures between about 1 and 200 bar in an inert solvent, for example an alcohol, such as methanol, ethanol or isopropanol, an ether, such as tetrahydrofuran (THF) or dioxane, an ester, such as ethyl acetate, a carboxylic acid, such as acetic acid, or a hydrocarbon, such as cyclohexane. Suitable catalysts are advantageously noble metals, such as Pt or Pd, which can be employed in the form of oxides (for example PtO2 or PdO), on a support (for example Pd on charcoal, calcium carbonate or strontium carbonate) or in finely divided form.
Ketones can also be reduced by the methods of Clemmensen (using zinc, zinc amalgam or tin and hydrochloric acid, advantageously in aqueous-alcoholic solution or in the heterogeneous phase with water/toluene at temperatures between about 80xc2x0 C. and 120xc2x0 C.) or Cagliotti [by reacting the tosyl hydrazones with sodium borohydride, sodium cyanoborohydride or catecholborane (Org. Synth. 52, 122 (1972))] to give the corresponding compounds of the formula I containing alkyl groups and/or xe2x80x94CH2CH2xe2x80x94 bridges.
Reductions using complex hydrides are also possible. For example, arylsulfonyloxy groups can be removed reductively using LiAlH4, in particular p-toluenesulfonyloxymethyl groups can be reduced to methyl groups, advantageously in an inert solvent, such as diethyl ether or THF, at temperatures between about 0 and 100xc2x0 C.
The esters according to the invention can be prepared by esterifying corresponding carboxylic acids (or reactive derivatives thereof) using alcohols or phenols (or reactive derivatives thereof).
Suitable reactive derivatives of said carboxylic acids are, in particular, the acid halides, especially the chlorides and bromides, furthermore the anhydrides, for example including mixed anhydrides, azides or esters, in particular alkyl esters having 1-4 carbon atoms in the alkyl group.
Suitable reactive derivatives of said alcohols or phenols are, in particular, the corresponding metal alkoxides or phenoxides, preferably of an alkali metal, such as Na or K.
The esterification is advantageously carried out in the presence of an inert solvent. Highly suitable solvents are, in particular, ethers, such as diethyl ether, di-n-butyl ether, THF, dioxane or anisole, ketones, such as acetone, butanone or cyclohexanone, amides, such as DMF or hexamethylphosphoric triamide, hydrocarbons, such as benzene, toluene or xylene, halogenated hydrocarbons, such as tetrachloromethane or tetrachloroethylene, and sulfoxides, such as dimethyl sulfoxide or sulfolane. Water-immiscible solvents may advantageously be used at the same time for removal by azeotropic distillation of the water formed in the esterification. An excess of an organic base, for example pyridine, quinoline or triethylamine, as solvent for the esterification may sometimes also be used. The esterification can also be carried out in the absence of a solvent, for example by simply heating the components in the presence of sodium acetate. The reaction temperature is usually between xe2x88x9250xc2x0 and +250xc2x0 C., preferably between xe2x88x9220xc2x0 C. and +80xc2x0 C. At these temperatures, the esterification reactions are generally complete after 15 minutes to 48 hours.
In detail, the reaction conditions for the esterification depend substantially on the nature of the starting materials used. For example, a free carboxylic acid is generally reacted with a free alcohol or phenol in the presence of a strong acid, for example a mineral acid, such as hydrochloric acid or sulfuric acid. A preferred reaction procedure is to react an acid anhydride or in particular an acid chloride with an alcohol, preferably in a basic medium, suitable bases being, in particular alkali metal hydroxides, such as sodium hydroxide or potassium hydroxide, alkali metal carbonates or hydrogencarbonates, such as sodium carbonate, sodium hydrogencarbonate, potassium carbonate or potassium hydrogencarbonate, alkali metal acetates, such as sodium acetate or potassium acetate, alkaline earth metal hydroxides, such as calcium hydroxide, or organic bases, such as triethylamine, pyridine, lutidine, collidine or quinoline. A further preferred embodiment of the esterification comprises first converting the alcohol or phenol into the sodium or potassium alkoxide or phenoxide, for example by treatment with ethanolic sodium hydroxide solution or potassium hydroxide solution, isolating the alkoxide or phenoxide and suspending it in acetone or diethyl ether with stirring together with sodium hydrogencarbonate or potassium carbonate, and treating the suspension with a solution of the acid chloride or anhydride in diethyl ether, acetone or DMF, advantageously at temperatures between about xe2x88x9225xc2x0 C. and +20xc2x0 C.
Ethers according to the invention are obtainable by etherifying corresponding hydroxyl compounds, preferably corresponding phenols, where the hydroxyl compound is advantageously first converted into a corresponding metal derivative, for example into the corresponding alkali metal alkoxide or alkali metal phenoxide by treatment with NaH, NaNH2, NaOH, KOH, Na2CO3 or K2CO3. This alkoxide or phenoxide can then be reacted with the corresponding alkyl halide, alkyl sulfonate or dialkyl sulfate, advantageously in an inert solvent, such as acetone, 1,3-dimethoxyethane, DMF or dimethyl sulfoxide, or alternatively in an excess of aqueous or aqueous-alcoholic NaOH or KOH, at temperatures between about 20xc2x0 C. and 100xc2x0 C.
The liquid-crystalline phases according to the invention consist of 2 to 25, preferably 3 to 15 components, including at least one compound of the formula I. Particularly preferred mixtures comprise one, two, three or four compounds of the formula I, preferably one or two compounds of the formula I. The other constituents are preferably selected from nematic or nematogenic substances, in particular known substances, from the classes consisting of the azoxybenzenes, benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoates, phenyl or cyclohexyl cyclohexanecarboxylates, phenylcyclohexanes, cyclohexylbiphenyls, cyclohexylcyclohexanes, cyclohexylnaphthalenes, 1,4-bis-cyclohexylbenzenes, 4,4xe2x80x2-bis-cyclohexylbiphenyls, phenyl- or cyclohexylpyrimidines, phenyl- or cyclohexyldioxanes, phenyl- or cyclohexyldithianes, 1,2-di-phenylethanes, 1,2-dicyclohexylethanes, 1-phenyl-2-cyclohexylethanes, optionally halogenated stilbenes, benzyl phenyl ethers, tolans and substituted cinnamic acids.
The most important compounds which are suitable as constituents of liquid-crystalline phases of this type can be characterized by the formula 2
Rxe2x80x2xe2x80x94Lxe2x80x94Gxe2x80x94Exe2x80x94Rxe2x80x3xe2x80x83xe2x80x832
in which L and E are each a carbocyclic or heterocyclic ring system from the group consisting of 1,4-disubstituted benzene and cyclohexane rings, 4,4xe2x80x2-disubstituted biphenyl, phenylcyclohexane and cyclohexylcyclohexane systems, 2,5-disubstituted pyrimidine and 1,3-dioxane rings, 2,6-disubstituted naphthalene, di- and tetrahydronaphthalene, quinazoline and tetrahydroquinazoline,
G is xe2x80x94CHxe2x95x90CHxe2x80x94 xe2x80x94N(O)xe2x95x90Nxe2x80x94
xe2x80x94CHxe2x95x90CYxe2x80x94 xe2x80x94CHxe2x95x90N(O)xe2x80x94
=xe2x80x94Cxe2x89xa1Cxe2x80x94 xe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94COxe2x80x94Oxe2x80x94 xe2x80x94CH2xe2x80x94Oxe2x80x94
xe2x80x94COxe2x80x94Sxe2x80x94 xe2x80x94CH2xe2x80x94Sxe2x80x94
xe2x80x94CHxe2x95x90Nxe2x80x94 xe2x80x94COO-Phe-COOxe2x80x94
or a Cxe2x80x94C single bond, where Y is halogen, preferably chlorine, or xe2x80x94CN, and Rxe2x80x2 and Rxe2x80x3 are alkyl, alkoxy, alkanoyloxy or alkoxycarbonyloxy having up to 18, preferably up to 8, carbon atoms, or one of these radicals is alternatively CN, NC, NO2, OCF3, CF3, F, Cl or Br.
In most of these compounds, Rxe2x80x2 and Rxe2x80x3 are different from one another, one of these radicals usually being an alkyl or alkoxy group. However, other variants of the proposed substituents are also common. Many such substances or mixtures thereof are commercially available. All these substances can be prepared by methods which are known from the literature.
Besides one, two, three, four or more compounds of the formula I, the preferred mixtures comprise, in particular, one or more compounds of the components listed below: 
in which
R* is as defined for R1 or R2 and is preferably straight-chain alkyl, alkoxy, vinyl, 1 E-alkenyl or 3 E-alkenyl,
Alkyl or Alkyl* are each, independently of one another, straight-chain alkyl having 1-6 carbon atoms, and
L is H or F.
The liquid-crystalline phases according to the invention comprise from about 0.1 to 99%, preferably 10 to 95%, of one or more compounds of the formula I. Additionally preferred liquid-crystalline phases are those which comprise 0.1-50%, particularly 0.5-30%, of one or more compounds of the formula I. Isotropic compounds of the formula I can also be used in the phases according to the invention.
The liquid-crystalline phases according to the invention are prepared in a manner which is customary per se. In general, the components are dissolved in one another, expediently at elevated temperature.
By means of suitable additives, the liquid-crystalline phases according to the invention can be modified in a manner such that they can be used in all types of liquid-crystal display elements which have been disclosed hitherto.
Additives of this type are known to those skilled in the art and are described in detail in the literature. For example, conductive salts, preferably ethyldimethyldodecylammonium 4-hexyloxybenzoate, tetrabutylammonium tetraphenylborate or complex salts of crown ethers (cf., for example, I. Haller et al., Mol. Cryst. Liq. Cryst. Volume 24, pages 249-258 (1973)) can be added in order to improve the conductivity, dichroic dyes can be added to produce coloured guest-host systems or substances can be added to modify the dielectric anisotropy, the viscosity and/or the orientation of the nematic phases. Substances of this type are described, for example, in DE-A 2,209,127, 2,240,864, 2,321,632, 2,338,281, 2,450,088, 2,637,430, 2,853,728 and 2,902,177.
In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals CnH2n+1 and CmH2m+1 are straight-chain alkyl radicals containing n or m carbon atoms respectively. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is given. In each individual case, the acronym for the parent structure is followed, separated by a hyphen, by a code for the substituents R1, R2, L1 and L2:
Tables A and B show preferred mixture components in the liquid-crystalline mixtures according to the invention.
Particularly preferred mixtures according to the invention, in particular for TFT-ECB applications, comprise, besides one or more compounds of the formula I, one, two, three or four compounds of the formula PCH-nmFF, PCH-nOmFF, CCP-nmFF and/or CCP-nOmFF. Preference is furthermore given to mixtures which comprise, besides one or more compounds of the formula I, one, two, three or four neutral compounds of the formula PCH and/or CCH in which R1 and/or R2 are alkyl or alkoxy having 1 to 5 carbon atoms.
The examples below are intended to illustrate the invention without representing a limitation. Above and below, percentages are per cent by weight. All temperatures are given in degrees Celsius. m.p. denotes melting point, cl.p.=clearing point. Furthermore, C=crystalline state, N=nematic phase, S=smectic phase and I=isotropic phase. The figures between these symbols are the transition temperatures. An denotes the optical anisotropy (589 nm, 20xc2x0 C.), and the viscosity (mm2/sec) was determined at 20xc2x0 C.
xe2x80x9cConventional work-upxe2x80x9d means that water is added if necessary, the mixture is extracted with dichloromethane, diethyl ether, methyl tert-butyl ether or toluene, the organic phase is separated off, dried and evaporated, and the product is purified by distillation under reduced pressure or crystallization and/or chromatography. The following abbreviations are used:
BuLi Butyllithium
DAST Diethylaminosulfur trifluoride
DCC Dicyclohexylcarbodiimide
DDQ Dichlorodicyanobenzoquinone
DIBALH Diisobutylaluminium hydride
KOT Potassium tertiary-butoxide
THF Tetrahydrofuran
pTsOH p-Toluenesulfonic acid
TMEDA Tetramethylethylenediamine
MOST Morpholinosulfur trifluoride